FRITS OVERMARS PDF

Voir le deal. I think to be courteous to the people answering questions that it would be best if any newcomers take the time to read this thread from start to finish. It is some of the best reading on the planet and is like an encyclopedia of information about real world 2 stroke performance. I think Jan and Frits are becoming somewhat tired of re-answering questions. That said I hope that Mr Thiel and Mr Overmars don't mind that I have printed this thread in its entirity and have a copy in my library, next to the porcelain throne for reference.

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Voir le deal. I think to be courteous to the people answering questions that it would be best if any newcomers take the time to read this thread from start to finish. It is some of the best reading on the planet and is like an encyclopedia of information about real world 2 stroke performance. I think Jan and Frits are becoming somewhat tired of re-answering questions.

That said I hope that Mr Thiel and Mr Overmars don't mind that I have printed this thread in its entirity and have a copy in my library, next to the porcelain throne for reference. Sorry to disturb you, gentlemen! As you probably don't know, there is an automatic process to split a topic after 50 pages Most considerate of you, Howard. As you may have noticed I stopped answering questions four days ago and I am glad Jan took over. Citation : That said I hope that Mr Thiel and Mr Overmars don't mind that I have printed this thread in its entirity and have a copy in my library, next to the porcelain throne for reference.

Everybody needs some form of paper next to that throne and I feel honoured that you choose our life's work. It was spelt out several times that it's the pipe that pulls gas through the trannsfers, not the crankcase compression which pushes. On the other hand, I'm surprised at the anti-hydroforming suggestion : I've always viewed multi-stage diffusers as an approximation to an exponential horn, which gives an equal proportion of energy returned per unit time.

Has it been discussed somewhere in the thread previously, or is it a new claim? I've read pretty much all books that you can get, but this 51 pages feature more knowledge then all of those together. Thats sth I learned from here that the pipe sucks, but somehow i think there is and from tested it seems like there is a limit. You can also open a new topic here Thiel or mr. Is it possible that too big clearance between the crankshaft webs and crankcase could reduce performance?

I lately built an engine that has about 2mm of space between the crankshaft webs and crankcase, and the engine compared to other similar engines has substantially lower performance. It's a small 70cc engine, the crankshaft webs diameter is 71mm and crankcase is Hi Marko, it is interesting. The same thing is with Malossi crankshaft for Piaggio crankcase which has 2 mm less in webs diameter than original one to fit also older crankase so that means 4mm space between webs and crankcase like in your case and the power is not lower, but the webs on Malossi are fuller and crankshaft is heavier.

Thanks for your answer. Just removing 0,5mm gave less power! I do not know the answer to this question, because I never tried this!

Normally high crankcase cv. I think there is also a limit of how low you can go, but I do not know where this limit is! The flow from the inlet into the transfer ducts is very important but little known I think. It is impossible to simulate on a flow bench! And difficult to understand, with the moving conrod, piston and crankshaft. One thing I am sure about is: always give priority to flow over crankcase volume! The same thing is with Malossi crankshaft for Piaggio crankcase which has 2 mm less in webs diameter than original one.

A smaller crank web diameter means less material around the big end hole, a weaker press fit and a flabbier crankshaft. That may well nullify the benefits of a larger crankcase volume. So if you have a crankshaft with a 40 mm stroke and a 16 mm big end pin, the web diameter should be at least 72 mm. Our engines have typically a 2 mm clearance give or take for casting flaws between the crank webs and the case. We tried closing the clearance between the crank and the case to.

It was a lot of work and did reduce the volume of the cases substantially but resulted in less power. We then went the otherway and machined a crankshaft down adding an extra 2 mm clearance on another case which gave now 4 mm clearance. It resulted in a small gain in power. My dilema is that we do not know if the reduction in power was a result of added viscous drag or as a result of tighter volume and was the increasein power as a result of the fact that inlet flow was less impeded by the crank web or was the added volume the cause.

My recommendation for crankcase volume is bigger is better than smaller in moderation anything that adds extra airflow always adds power. A good airflow turbine is a great tool.

While on the subject of crankshafts I have noticed that the RSA uses a full circle crankshaft and there is no indication of what we call "turbo" machining on the webs. Basically it is machined grooves in the crank webs that act as a sort of fan blades creating forced flow out to the transfers. We have tested torbo crankshafts and they seem to give a small increase in power and an increase in airflow. Jan did you ever try a turbo crankshaft in the RSA and what were the results?

When I read this, a question arises to me. I once heard, that the surface of the last part of the inlet tract should not be bigger than the carburetor surface or the effectice surface of the reed valve in a reed valve engine But if flow from the inlet into the transfers is really as important as you say, then maybe a big radius would make sense!?

I have made a little sketch to show what I mean. Frits, Jan, Howard, or anyone, could you say a few words about how to size the stinger diameter? My experience is that as you go smaller everything gets better especially top end power and then at some point the piston gets too hot or else you get into too much detonation. I always considered the stinger last, after designing all the rest of the pipe: is this a bad idea? Frits, I imagine this was difficult with the.

I prefer not to talk about a stinger because that would imply a tube of some length. And length is not the issue here; the issue is flow resistance and diameters are far more decisive than lengths in that respect. Therefore I always apply a venturi as the flow-controlling element at the end of the reflector cone. It can be made exchangeable which allows experiments, and because the flow is controlled by the venturi, it allows for a tailpipe with a much larger diameter that is less susceptible to dents and deposit build-up.

It also allows for a longer tailpipe in case you need to move a silencer out of the way. You may be wondering about waves running up and down the tailpipe. Those waves will never be able to influence the events in the exhaust pipe because the flow through a correctly dimensioned venturi is sonic.

Those waves are like birds flying against a storm: they will lose all their energy but they will never make any progress. Your experience is correct: more flow restriction causes a higher average pressure in the exhaust pipe, hence less expansion and a higher average temperature of the gas in the pipe, and hence a higher speed of sound and a higher resonance rpm. And if you create too much flow restriction, the exhaust gases in the cylinder will not completely have left the building by the time the transfer ports open; they will enter the transfer ducts and that flow of hot gases will severely heat up the cylinder, the piston and the fresh mixture in the transfer ducts.

The required flow restriction depends solely on the amount of generated exhaust gas, and that is directly proportional to the generated horsepower. I am almost certain that somewhere on this forum I already posted a simple exhaust concept that included the venturi calculation. But I can't find it through the search option forum search options and I never seem to get along so here it is once more, together with a sketch of the exchangeable venturi red in the sketch.

Brian, you are right about the flow restriction influencing the mixture strength of the MB40 model aero engine. For those of you who are less familiar with it: these model airplanes keep their fuel in a bladder.

That bladder is contained in a tank, and the exhaust pipe pressure is fed to that tank. The pipe pressure tries to squeeze the bladder and the fuel is pushed to the inlet port via a regulator needle. But since the needle setting is optimized for each flight anyway, it doesn't matter if the fuel pressure is not always the same. It is called Dtailrestrictor It was real. That bike must be about ten years old now.

It never got beyond the prototype stage. Aprilia ceased developing street two-strokes a long time ago. I know a hell lot of people from MX including myself that would love to go back to twostrokes.

Once you have blown a fourstroke, which will happen if u ride it above hobbyspeed, your wallet will be damn empty and the garbage bin full of camchain, valves, a cylinder, maybe a cylinder head, a piston and probably a crank. Can you show us a picture of a "turbo-crank", Howard? Daniel A. As far as I know the flow restrictor was first used by Helmut Fath. Page 1 sur Sujets similaires. Permission de ce forum:.

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He told me he built an optimized pipe designed by the Mota software and it didn't give him good power. The Mota waves didn't change from single to triple cone diffuser. At least the diffuser wave should change but it didn't so Mota doesn't use the detailed wave generation method that I use, otherwise the wave peak would be more to the left with a single cone diffuser. ECcalc says the diffuser wave should start at 1. Mota shows a high baffle wave peak but the contrary 2nd generation diffuser wave overlaps it which would lower the peak but Mota doesn't show that lowering.

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If you are interested in 2 stroke engines and bikes please follow this blog or check back regularly as I will continue to post information and projects as time goes on. Sunday, March 11, Epoxy vs Ethanol Test. Been a while since posting anything or tackling projects. Between full time work and school followed by the cold winter weather its been slow here. I did a test on a well known 2 part epoxy putty that was recommend on numerous sites for port modifications, especially among 4 stroke car guys. The epoxy product is called A Splash Zone Epoxy. It seems to mainly be aimed at the marine industry for boat hull repair.

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